Pseudo Wire Emulation Edge-to-Edge (PWE3) provides a tunnel on a packet switched network (IP/Multiple Protocol Label Switching (MPLS)), so as to emulate a layer 2 Virtual Private Network (VPN) protocol of some services (e.g., Frame Relay (FR), Asynchronous Transfer Mode (ATM), Ethernet, Time Division Multiplex Synchronous Optical Network/Synchronous Digital Hierarchy (TDM SONET/SDH)), and a traditional network and the packet switched network can be interconnected through the protocol, thereby realizing the resource sharing and network expansion.
The PWE3 is an extension to an L2VPN Martini protocol, in which a function of Multi Segment Pseudo Wires (MSPW) is added and a networking mode is extended. The MSPW means that multiple Pseudo Wires (PW) exist between a User-end Provider Edge (U-PE, also called as a user access device) and a U-PE. A forwarding mechanism of a U-PE in multiple hops is the same as a forwarding mechanism of a U-PE in a single hop, except it is needed to perform label switching of a PW Label layer on a Switching PE (S-PE) in the multi-hop forwarding. In most situations, the single-hop PW can meet practical requirements, but under the following several situations the single-hop PW cannot meet the requirements and the multi-hop PW needs to be adopted: two PEs are not in a same Autonomous System domain, and a signaling connection or a tunnel cannot be established between the two PEs; signalings on the two PEs are different, for example, one end runs a Label Distribution Protocol (LDP) and the other end runs a Resource Reservation Protocol (RSVP); if access devices can run the MPLS but have no capability to establish a large number of LDP sessions, then User Facing Provider Devices (UFPE) can be taken as U-PEs at this point, and high performance devices S-PEs can be taken as switching nodes of the LDP sessions, similar to a signaling reflector.
The function of MSPW can reduce the requirement for the number of LDP connections supported by the access devices, i.e., can reduce overheads of the LDP sessions of the access nodes; the multi-hop access nodes meet a convergence function of the PW, which makes the network more flexible and appropriate for grading (access, convergence and core). More telecom low-speed Time Division Multiplex (TDM) interfaces are supported.
In an MSPW network in the PWE3 application, when a failure occurs in a device between intermediate S-PEs, it causes that data messages cannot be forwarded successfully, so as to affect the quality of service.